Fabricating nano-scale devices that function as ohmic contacts and have low resistance has been a challenge to the semiconductor industry. Conventional low resistance ohmic contacts are made of metal suicides formed on heavily doped semiconductor regions. The contact resistance is inversely proportional to contact area. In nano-scale devices, the contact area is on the order of nanometer or smaller and, thus, contact resistance limits performance.
Carbon nanotube structures are finding their way into nano-scale devices due to their unique electron transport properties. The carbon nanotubes may be metallic or semiconductive. Carbon nanotubes have high current densities, such as up to about 109 A/cm2. This and other properties of the carbon nanotubes make them ideal candidates for use in molecular-scale electronic devices. Due to their low dimensionality, structural symmetry and electronic properties, carbon nanotubes are being explored for use in transistors, interconnects and switches for non-volatile memory applications.
Integrated circuits combining carbon nanotubes with a silicon metal-oxide semiconductor field-effect transistor (MOSFET) switching circuit have been formed by growing the carbon nanotubes onto the integrated circuit at predefined locations. Conventionally, the carbon nanotubes are positioned on gate oxides and contacted by polycrystalline thin film metal electrodes or are overgrown epitaxially by high-k dielectrics using atomic layer deposition.
Presently, the manufacturing of carbon nanotube memory devices depends on a so-called “top-down” fabrication technique. For example, a film including a monolayer of nanotubes is deposited on an electrically conductive material using a spin-on technique and is lithographically patterned to make columns and rows of the nanotubes. The 1 nm to 2 nm-thick, patterned nanotube is interconnected with complementary metal-oxide semiconductor (CMOS) circuitry.
Semiconductor structures have also been formed including so-called “ribbons” of carbon nanotubes that are suspended over a carbon substrate. In the “off” state, the ribbon of carbon nanotubes does not touch the carbon substrate, and electricity does not flow between an interconnect suspending the ribbon. In the “on” state, the carbon nanotubes bend downward and adhere to the carbon substrate through van der Waals forces, enabling electricity to flow between the interconnect. However, bit failure is a common occurrence with the above-mentioned techniques because reliable contact between electrodes and the carbon nanotubes has not been achieved.
Semiconductor structures with improved control structures are desired, as are methods of forming such semiconductor structures.